1. Field of the Invention
The present invention relates to a card-like portable memory medium, a so-called IC card made by laminating integrated circuit chips such as a memory circuit and a microcomputer to store various kinds of information, and particularly to an improvement of a protective circuit connected to connection terminals of the portable memory medium.
2. Description of the Prior Art
An IC card complies with the ISO standards for its various performances. Among the ISO standards, there is an electrostatic test. The IC card is carried by a person under various environmental conditions and frequently subjected to an external large static electricity. The IC card has a plurality of connection terminals to be electrically connected to an external apparatus such as an automatic transaction machine (ATM) installed in banking agencies, and the connection terminals are internally connected to IC chips such as a microcomputer and a memory circuit. If these internal IC chips are subjected to the static electricity, they may be destroyed.
An object of the electrostatic test of the ISO standards is to prevent the IC card from easily being destroyed by such external static electricity. The IC card has a common connection terminal, i.e., a grounding terminal (GND) for supplying common potential, and a plurality of independent connection terminals such as a power source voltage terminal (Vcc), an input/output terminal, a reset signal terminal, and a clock signal terminal. The electrostatic test of the ISO standards applies 1,500 volts charged in a capacitor of 100 pF to positions between the grounding terminal and the respective independent connection terminals through a resistor of 1,000 ohms to see whether or not the internal circuits of the IC card can withstand the applied electricity.
The IC card is equipped with, therefore, a protective circuit which can withstand the test and protect the internal circuits of the IC card against external static electricity.
FIG. 1A is a circuit diagram showing an IC card having a prior art protective circuit. The IC card has connection terminals 1a to 1h for connecting the IC card to an external apparatus. The connection terminals are, for instance, a power source voltage terminal (Vcc) 1a, a grounding terminal (GND) 1b, a reset signal terminal (RST) 1c a clock signal terminal (CLK) 1e, an input/output terminal (I/O) 1f etc. The respective connection terminals are connected through a protective circuit 3 to integrated circuit chips such as a CPU 5 comprising a microprocessor, and a memory circuit 7 comprising an E.sup.2 PROM.
The protective circuit 3 has four zener diode blocks 3a, 3b 3c, and 3d arranged in parallel with each other, each comprising a pair of zener diodes connected in series in reverse polarity. Each of the zener diode blocks 3a, 3b, 3c, and 3d has a first and a second zener diodes, cathodes of which are internally interconnected, and is formed as a single semiconductor element of a rectangular parallelepiped. The four zener diode blocks constitute the protective circuit 3.
Anodes of the first zener diodes of the zener diode blocks 3a, 3b, 3c and 3d are connected commonly and connected to the grounding terminal 1b. An anode of the second zener diode of the first zener diode block 3a is connected to the power source voltage terminal 1a, an anode of the second zener diode of the second zener diode block 3b to the reset signal terminal 1c, an anode of the second zener diode of the third zener diode block 3c to the clock signal terminal 1e, and an anode of the second zener diode of the fourth zener diode block 3d to the input/output terminal 1f.
In the IC card having the protective circuit 3 of the above arrangement, when static electricity or an extremely high voltage such as 1,500 volts in the electrostatic test of the ISO standards is applied between the grounding terminal 1b and the respective terminals i.e., the power source voltage terminal 1a, reset signal terminal 1c, clock signal terminal 1e, and input/output terminal 1f, the zener breakdown or the avalanche breakdown of the zener diodes is caused to prevent the internal circuits connected to the respective connection terminals from being destroyed.
Namely, the zener diode blocks 3a, 3b, 3c, and 3d are connected between the grounding terminal 1b and the connection terminals 1a, 1c, 1e, and 1f respectively, and, when a high voltage of, for instance, 1,500 volts of any polarity is applied between the terminals, one of the zener diodes of each of the zener diode blocks is in the forward direction with respect to the applied voltage. Accordingly, most of the high voltage is given to the other zener diodes of the zener diode blocks, and the other zener diodes will demonstrate the zener breakdown or the avalanche breakdown with respect to the high voltage to decrease the applied high voltage to a constant voltage of the zener diodes. In this way, even if a high voltage is applied between the terminals, the voltage between the terminals is suppressed to the constant voltage of the zener diodes, for instance 6.5 volts, so that the high voltage is not applied to the internal circuits connected to the terminals, thereby preventing the internal circuits from being destroyed by the high voltage.
FIG. 2A is a circuit diagram showing another example of the prior art protective circuit used for an IC card. A circuit constitution of this protective circuit 30 is similar to that of the protective circuit 3 shown in FIG. 1A. A difference between them is that the protective circuit 3 shown in FIG. 1A has four separate zener diode elements each having a rectangular parallelepiped with a pair of zener diodes, and the protective circuit 30 shown in FIG. 2A comprises eight zener diodes which are formed by diffusion on a single semiconductor chip as shown in FIG. 2B. A circuit constitution inside the IC card connected to the protective circuit 30 is the same as that of the circuit shown in FIGS. 1A and 1B.
As shown in FIG. 2B, by forming the protective circuit 30 of eight zener diodes as a single semiconductor chip, the protective circuit may easily be assembled in the IC card.
Since the protective circuit 3 shown in FIG. 1A is made by connecting four separate zener diode block elements each having a rectangular parallelepiped, the number of assembling processes is large, deteriorating the productivity, and the size is hardly minimized, causing a problem that it is vulnerable to a bending test of IC card.
Although the protective circuit 30 shown in FIGS. 2A and 2B is constituted by a single semiconductor chip, it requires eight zener diodes and relatively enlarges the size of its structure. Therefore, similar to the protective circuit 3 shown in FIG. 1A, the electrostatic capacitance of a junction of a zener diode of the prior art protective circuit is increased, causing problems that it may exceed the maximum capacitance, for instance 30 pF, specified by the ISO standards, and that it is vulnerable to the bending test of IC card due to the large structure size. Since the IC card is carried by a person, it should be strong to a certain extent against bending. To check the bending strength, the ISO standards include a bending test in which a transversal twist, etc., of the IC card are checked. Since the protective circuits shown in FIGS. 1 and 2 have relatively large structures, a large bending force is applied to elements constituting the protective circuits, destroying the elements.